1. Field of the Invention
The present invention relates to a three-band switching oscillator for use in portable telephones or the like, which is designed to output an oscillation signal in any one of three frequency bands.
2. Description of the Related Art
The recent oscillators have been designed to make oscillation in a plurality of frequency bands. FIG. 5 is an illustration of an arrangement of a conventional three-band switching oscillator handling three systems, for example, DCS (cellular telephone using 1800 MHz band), PCS (cellular telephone using 1900 MHz band) and GSM (cellular telephone using 900 MHz band). In addition, this takes four modes: output of an oscillation signal in any one of frequency bands and non-output of an oscillation signal in these frequency bands.
In FIG. 5, a first voltage-controlled oscillator 31 is composed of a first oscillation transistor 32 and a first resonance circuit 33 connected to the base thereof, and serves as a common collector type oscillator. The first resonance circuit 33 includes a first varactor diode 33a and resonant lines 33b and 33c connected in series to each other, while a switching diode 33d is connected in parallel with one resonant line 33c. The anode of the first varactor diode 33a is grounded while the cathode thereof is connected to a control terminal 34. Moreover, the cathode of the switching diode 33d is in a grounded condition.
An oscillation signal is outputted from the emitter of the first oscillation transistor 32 and is inputted to the base of a first amplification transistor 35a constituting a first amplifier 35. In addition, the oscillation signal amplified is outputted from the collector of the first amplification transistor 35a. 
The first amplification transistor 35a and the first oscillation transistor 32 are connected in series with respect to a power supply, and a power supply voltage Vb is applied to the collector of the first amplification transistor 35a, while a voltage is applied through the emitter thereof to the collector of the first oscillation transistor 32. In addition, resistors 36, 37 and 38 for setting a bias voltage are connected to the base of the first amplification transistor 35a and the base of the first oscillation transistor 32, and the resistor 36 is connected to the emitter of the first switching transistor 39. The power supply voltage Vb is given to the collector of the first switching transistor 39, while the base thereof is connected to as first switching terminal 40. The anode of the switching diode 33d is connected to a second switching terminal 41.
Meanwhile, a second voltage-controlled oscillator 51 is composed of a second oscillation transistor 52 and a second resonance circuit 53 connected to the base thereof, and acts as a common collector type oscillator. The second resonance circuit 53 includes a second varactor diode 53a, a resonant line 53b, and other elements. The anode of the second varactor diode 53a is grounded, while the cathode thereof is connected to the control terminal 34.
An oscillation signal is outputted from the second oscillation transistor 52 and is inputted to the base of a second amplification transistor 54a constituting a second amplifier 54. In addition, the oscillation signal amplified is outputted from the collector of the second amplification transistor 54a. 
The second amplification transistor 54a and the second oscillation transistor 52 are connected in series with respect to the power supply, and the power supply voltage Vb is applied to the collector of the second amplification transistor 54a, while a voltage is supplied through the emitter thereof to the collector of the second oscillation transistor 52. In addition, resistors 55, 56 and 57 for setting a bias voltage are connected to the base of the second amplification transistor 54a and the base of the second oscillation transistor 52, and the resistor 55 is connected to the emitter of a second switching transistor 58. The power supply voltage Vb is given to the collector of the second switching transistor 58, and the base thereof is connected to a third switching terminal 59.
In the above-mentioned arrangement, a high-level or low-level switching voltage coming from the body side of the portable telephone is applied to the first switching terminal 40, the second switching terminal 41 and the third switching terminal 59. Referring to FIG. 6, a description will be given of an operation of each of the voltage-controlled oscillators 31 and 51 based on;this switching voltage. In the description which will be given with reference to FIG. 6, the first to third switching terminals 40, 41 and 59 are taken as SW1, SW2 and SW3, respectively, while the high-level and low-level switching voltages are taken as (H) and (L), respectively. In addition, the first voltage-controlled oscillator 31 is taken as VCO1 and the second voltage-controlled oscillator 51 is taken as VCO2.
First, when the SW1 is at (L), the first switching transistor 39 turns off to stop the bias voltage supply to the base of the first oscillation transistor 32, thereby making the first voltage-controlled oscillator 31 inoperative. Contrary to this, when the SW1 is at (H), it becomes operative. In addition, when the SW2 is at (L) in the operative condition, the switching diode 33d turns off, and the VCO1 oscillates in a first frequency band which is low in frequency. This frequency band is for use in the DCS system. On the other hand, when the SW2 is at (H), the switching diode 33d turns on, so the VCO1 oscillates in a second frequency band which is high in frequency. This frequency band is for use in the PCS system.
In like manner, when the SW3 is at (L), the second switching transistor 58 turns off to stop the bias voltage supply to the base of the second oscillation transistor 52, so the second voltage-controlled oscillator 51 becomes inoperative. Contrary to this, when the SW2 is at (H), it becomes in an actuated condition, and the oscillation takes place in a third frequency band. This frequency band is available for the GSM system.
That is, in the above-described operation, a high-level or low-level switching voltage is inputted to the first to third switching terminals 40, 41 and 59, and eight switching modes are attainable according to combinations thereof. FIG. 6 shows these switching modes, and of these, the modes No. 1 and No. 2 are the same operational contents and the modes No. 3 and No. 4 are the same operational contents, so only one modes are put to use. In addition, since the first voltage-controlled oscillator 31 and the second voltage-controlled oscillator 51 are not put to use at the same time, the modes No. 7 and No. 8 are not employable.
In the above-described conventional three-band switching oscillator, the three switching terminals (40, 41, 59) are used in order to obtain four necessary switching modes. The most important point to an oscillator for use in a portable telephone is size reduction, and hence, various improvements have been made in order to realize the size reduction. An decrease in number of terminals contributes greatly to the size reduction; however, an approach to decrease the number of terminals has not been made until now.
Furthermore, in the conventional three-band switching oscillator, since the binary switching voltages are applied to the three switching terminals, unnecessary switching modes come about. This complicates the circuit on the body side of a portable telephone which generates the switching voltages.
Accordingly, it is an object of the present invention to provide a three-band switching oscillator capable of decreasing the number of switching terminals for achieving the size reduction and of simplifying the arrangement of the body side of a portable telephone which generates switching voltages.
For achieving this purpose, in accordance with the present invention, there is provided a three-band switching oscillator comprising a first voltage-controlled oscillator switched to make oscillation in a first or second frequency band, a second voltage-controlled oscillator made to make oscillation in a third frequency band, and a switching circuit for switching the first and second voltage-controlled oscillators to an operative condition or an inoperative condition and further for switching an oscillation frequency band of the first voltage-controlled oscillator, the switching circuit including first switch means for supplying a current to place the first voltage-controlled oscillator into the operative condition, second switch means for supplying a current to place the second voltage-controlled oscillator into the operative condition, and first and second switching terminals for receiving switching voltages, wherein the first switch means is placed into an open or closed condition in accordance with a low-level or high-level switching voltage inputted to the first switching terminal and the oscillation frequency band is switched in accordance with a high-level or low-level switching voltage inputted to the second switching terminal, while only when the high-level switching voltage is inputted to the second switching terminal, the second switch means is placed into an open condition by the high-level switching voltage inputted to the first switching terminal and placed into a closed condition by the low-level switching voltage inputted thereto, and when the low-level switching voltage is inputted to the second switching terminal, the second switch means is placed into the open condition irrespective of the switching voltage inputted to the first switching terminal.
In addition, a first oscillation transistor is provided in the aforesaid first voltage-controlled oscillator and a second oscillation transistor is provided in the second voltage-controlled oscillator, while the first switch means is placed in series to a first resistor circuit for applying a bias voltage to the base of the first oscillation transistor, and the second switch means is placed in series to a second resistor circuit for applying a bias voltage to the base of the second oscillation transistor.
Still additionally, the first switch means is constructed with a first NPN transistor whose collector is connected to a power supply voltage applying terminal and whose emitter is connected to the first resistor circuit, while the second switch means is constructed with a second NPN transistor whose collector is connected to the power supply voltage applying terminal and a first PNP transistor whose emitter is connected to the emitter of the second NPN transistor and whose collector is connected to the second resistor circuit, with the bases of the first NPN transistor and the first PNP transistor being connected to the first switching terminal and the base of the second NPN transistor being connected to the second switching terminal.
Moreover, the first voltage-controlled oscillator includes a resonant line and a switching diode connected in parallel with the resonant line, with the cathode of the switching diode being grounded and the anode thereof being connected to the emitter of the second NPN transistor.
Still moreover, a first oscillation transistor is provided in the first voltage-controlled oscillator and a second oscillation transistor is provided in the second voltage-controlled oscillator, while the first switch means is interposed between the emitter of the first oscillation transistor and the ground and the second switch means is placed in series to a second resistor circuit for applying a bias voltage to the base of the second oscillation transistor.
In addition, the first switch means is constructed with a third NPN transistor whose collector is connected to the emitter side of the first oscillation transistor and whose emitter is connected to the ground side, and the second switch means is constructed with second and third PNP transistors whose emitters are connected to a power supply voltage applying terminal, with the collector of the third PNP transistor being connected to the second resistor circuit and the base of the third NPN transistor being connected to the first switching terminal, while the collector of the second PNP transistor and the base of the third PNP transistor being connected to each other and connected through a resistor to the first switching terminal and the base of the second PNP transistor being connected to the second switching terminal.
Still additionally, the first voltage-controlled oscillator includes a resonant line and a switching diode connected in parallel with the resonant line, with the cathode of the switching diode being grounded and the anode thereof being connected to the second switching terminal.
Moreover, the difference in center frequency between the first frequency band and the second frequency band is set to be smaller than the difference in center frequency between the first frequency band and the third frequency band and the difference in center frequency between the second frequency band and the third frequency band.